1. Field of the Disclosure
The present disclosure relates generally to an optical scanning system in an imaging apparatus, and particularly to such a system utilizing a scan lens design and arrangement thereof which allow for a more compact scanning unit.
2. Description of the Related Art
In various imaging devices which utilize light to form images, optical scanning systems are typically employed to scan modulated light beams from one or more light sources onto at least one target surface on which images are to be formed. In an electrophotographic imaging device, for example, an optical scanning system typically includes a scanning mirror which reflects a modulated light beam towards a plurality of optical components. Such optical components may include lenses and mirrors which direct and focus the reflected light beam to form light spots upon a surface of a photosensitive member. As the scanning mirror moves, either in a reciprocating manner as with the case of a torsion oscillator or rotationally as with the case of a polygon mirror, the light beam reflected thereby is scanned across each of the optical components of the optical scanning system. Ultimately, the light beam impinges and is swept across the photosensitive member, which may itself be rotating, as scan lines so as to form latent images thereon.
A color laser printer, for example, may have four laser beam channels in its laser scanning unit (LSU), one for each of cyan, magenta, yellow, and black color planes. Scan lenses are used to focus the laser beams into small spot sizes on photosensitive members across all scan positions. In addition, the scan lenses keep a linear spot velocity during scanning and minimize the process and scan jitter induced by scanner mirror error. Scan lenses are complex optical components in the LSU and contribute a significant portion to the total size and cost of an LSU.
Some traditional optical designs for LSUs generally require one or two scan lenses per channel. Thus, the total quantity of scan lenses for all four channels for a color LSU may usually range from four to eight. Having such number of scan lenses may require a relatively large space requirement for the LSU. Moreover, the cost of the LSU also increases as the number of scan lenses increases.
In some existing designs, the number of scan lenses is reduced by allowing two channels to share one scan lens such that two laser beams enter the scan lens through opposite surfaces thereof. However, because two laser beams enter a single scan lens from opposite directions, the opposite lens surfaces must be symmetrical and the scan lens is typically large and thick in order to have a decent optical performance particularly on laser spot size. The cost of a plastic scan lens, for example, is mainly determined by the cycle time of the injection molding, and the cycle time is mainly determined by the thickness and size of the scan lens because a thicker lens requires much longer cooling time. As a result, the cost reduction due to the decrease in the quantity of scan lenses may be offset by increased cost per scan lens. Moreover, designs requiring two thick scan lenses may also add additional constraints on the optical layout of the LSU, such as requiring additional fold mirrors before the laser beams reach the scan lenses. This adds to the accumulated tolerances for the optical paths and makes it difficult to have precise optical alignment therein.
Accordingly, there is a need for an improved scanning unit which is more size and cost efficient.